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[103.229.16.4]) by smtp.gmail.com with ESMTPSA id d9443c01a7336-2bd5c0600b4sm48730865ad.28.2026.05.14.23.18.41 (version=TLS1_3 cipher=TLS_AES_128_GCM_SHA256 bits=128/128); Thu, 14 May 2026 23:18:44 -0700 (PDT) Message-ID: <6611f3ea-b70f-45c7-9332-235647643a49@oss.qualcomm.com> Date: Fri, 15 May 2026 14:18:40 +0800 Precedence: bulk X-Mailing-List: linux-block@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: MIME-Version: 1.0 User-Agent: Mozilla Thunderbird Subject: Re: [PATCH] sched: flush plug in schedule_preempt_disabled() to prevent deadlock To: Ming Lei , Peter Zijlstra Cc: Tejun Heo , Jens Axboe , linux-block@vger.kernel.org, linux-kernel@vger.kernel.org, Ingo Molnar , Juri Lelli , Vincent Guittot , Michael Wu , Thomas Gleixner References: <20260512085939.1107372-1-tom.leiming@gmail.com> <20260512120431.GC1889694@noisy.programming.kicks-ass.net> <20260512124021.GA2214256@noisy.programming.kicks-ass.net> <20260513073039.GG1889694@noisy.programming.kicks-ass.net> Content-Language: en-US From: Xiaosen In-Reply-To: Content-Type: text/plain; charset=UTF-8 Content-Transfer-Encoding: 7bit X-Proofpoint-Spam-Details-Enc: AW1haW4tMjYwNTE1MDA2MCBTYWx0ZWRfXzvZbJoL7TmKi 3t8W7lsxjxYX5fCvXsLi0sI4B3JceBWIHp6IORVX3bs1o36nHAuKxOt6H9RSP2bSk8Ko/W1LVmy S2rf/X7sbLRTH2o81OXohmN59ldmRIPqOzqqSxcZ9d2UqjDyVVfUycI+odBR9UpUL1PuhaecKUa UXHMqfSATWQdQeeKnrtQM8e3OgJFEWFEdtGG1R7lcOsZO3sZJfM1hBA+MoDwCGmWOkFozDBeXEe lHED7xlE3Qq71SBk1FRvrB1TT1ZRbBt4hVpNiBM6UIwe0N3m68qSdU3sms3tXQuFx5uvS3M+gLR OhneBbDJYQnmbT4eE0HpEidJAyQ2AXRcYRVcRQs7yTqAm95+LlLP4ABkTv8yohrcrBRnsUqn9cT dTb36JuFeApFmyT7ivJ15okCHL0J5+H8WMTQgFx7XrTINvmVYOzui+X1xNnTpt5qkhtIHBBOCiy cy58ZG6kmsIU1Ef+RPA== X-Authority-Analysis: v=2.4 cv=cZXiaHDM c=1 sm=1 tr=0 ts=6a06bac6 cx=c_pps a=JL+w9abYAAE89/QcEU+0QA==:117 a=nuhDOHQX5FNHPW3J6Bj6AA==:17 a=IkcTkHD0fZMA:10 a=NGcC8JguVDcA:10 a=s4-Qcg_JpJYA:10 a=VkNPw1HP01LnGYTKEx00:22 a=u7WPNUs3qKkmUXheDGA7:22 a=yx91gb_oNiZeI1HMLzn7:22 a=VwQbUJbxAAAA:8 a=1XWaLZrsAAAA:8 a=6Ujbnq6iAAAA:8 a=EUspDBNiAAAA:8 a=srUJ046Ut8OOLMdfYg8A:9 a=QEXdDO2ut3YA:10 a=324X-CrmTo6CU4MGRt3R:22 a=-sNzveBoo8RYOSiOai2t:22 X-Proofpoint-GUID: ZnHGHhf2AMrTl3deFsg5T1md81tNKFjx X-Proofpoint-ORIG-GUID: ZnHGHhf2AMrTl3deFsg5T1md81tNKFjx X-Proofpoint-Virus-Version: vendor=baseguard engine=ICAP:2.0.293,Aquarius:18.0.1143,Hydra:6.1.51,FMLib:17.12.100.49 definitions=2026-05-15_01,2026-05-13_01,2025-10-01_01 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 lowpriorityscore=0 spamscore=0 phishscore=0 suspectscore=0 adultscore=0 bulkscore=0 impostorscore=0 malwarescore=0 clxscore=1015 priorityscore=1501 classifier=typeunknown authscore=0 authtc= authcc= route=outbound adjust=0 reason=mlx scancount=1 engine=8.22.0-2605130000 definitions=main-2605150060 https://lore.kernel.org/lkml/20260427183848.698551-2-jstultz@google.com/ The above change can resolve the deadlock I reported before by setting task's state to TASK_RUNNING before switching context. There is the likely alternative fix below. https://lore.kernel.org/lkml/20260512025635.2840817-1-jstultz@google.com/ Regards, Xiaosen On 5/13/2026 4:08 PM, Ming Lei wrote: > On Wed, May 13, 2026 at 09:30:39AM +0200, Peter Zijlstra wrote: >> On Wed, May 13, 2026 at 10:07:03AM +0800, Ming Lei wrote: >>> On Tue, May 12, 2026 at 07:16:36AM -1000, Tejun Heo wrote: >>>> Hello, Ming. >>>> >>>> On Tue, May 12, 2026 at 11:45:14PM +0800, Ming Lei wrote: >>>>> On Tue, May 12, 2026 at 02:40:21PM +0200, Peter Zijlstra wrote: >>>>>> On Tue, May 12, 2026 at 02:04:32PM +0200, Peter Zijlstra wrote: >>>>>>> On Tue, May 12, 2026 at 04:59:39PM +0800, Ming Lei wrote: >>>>>>>> On preemptible kernels, a deadlock can occur when a task with plugged IO >>>>>>>> calls schedule_preempt_disabled(): >>>>>>>> >>>>>>>> schedule_preempt_disabled() >>>>>>>> sched_preempt_enable_no_resched() // preemption now enabled >>>>>>>> schedule() // <-- preemption can happen here >>>>>>>> sched_submit_work() >>>>>>>> blk_flush_plug() >>>>>>>> >>>>>>>> After sched_preempt_enable_no_resched() re-enables preemption, the task >>>>>>>> can be preempted (e.g., by a higher-priority RT task) before reaching >>>>>>>> blk_flush_plug() in sched_submit_work(). Since the task's state is >>>>>>>> already TASK_UNINTERRUPTIBLE (set by the mutex/rwsem slowpath caller), >>>>>>>> requests in current->plug remain unflushed for an unbounded time. >>>>>>>> >>>>>>>> If another task depends on those plugged requests to make progress (e.g., >>>>>>>> to release a lock the sleeping task needs), a deadlock results: >>>>>>>> >>>>>>>> - Task A (writeback worker): holds plugged IO, preempted before >>>>>>>> flushing, stuck on run queue behind higher-priority work >>>>>>>> - Task B: waiting for IO completion from Task A's plug, holds a lock >>>>>>>> that Task A needs to be woken up >>>> >>>> My memory is hazy around io_schedule but the above reads really weird to me. >>>> A task, regardless of its current state stays on the runqueue when >>>> preempted, so the condition is temporary. As soon as the preempted task can >>>> get CPU, it should unwind the situation. That's not a deadlock. Is the >>>> problem that there can be preemption-induced delay in flushing the plugs? >>> >>> IMO, preempting a `!TASK_RUNNING` task can be thought as effective sleep, >> >> No it cannot be. Preemption ignores task state. > > Yeah, I get similar conclusion too with AI's assistance. > > But both two reports show that the preempted task aren't switched back for > long enough time, can you share any idea for Michael & Xiaosen to investigate > further from scheduler side? > > https://lore.kernel.org/linux-block/20260417082744.30124-1-michael@allwinnertech.com/ > > https://lore.kernel.org/linux-block/5660795d-87de-46f5-add4-7729a02225ef@oss.qualcomm.com/ > > > Thanks, > Ming